Change a function system to be exclusive

[hymm]

Objective

• I was experimenting with a library for combining multiple systems into one system that could be scheduled, but there wasn't a way of marking a function system run exclusively.

Solution

• Add a method to SystemMeta to set the exclusive flag. Note that this PR doesn't add anyway to actually set the flag easily. You would have to create a custom system param to be able to set it.
• Add a system param to mark a function system to run exclusively.

Testing

• Haven't tested. The one worry would be how it interacts with the executor, but in theory this should just work, since the flag is the only way the executor knows that a system is exclusive or not.